1. Field of the Invention
The invention relates to an automatic transmission for a vehicle, and more particularly, to an automatic transmission for a vehicle of a type including a planetary gear set having at least four shift elements, and one planetary gear unit, in which rotation input to the planetary gear unit and is transmitted to the shift elements of the planetary gear set.
2. Description of Related Art
An automatic transmission of the above-described type is disclosed in Japanese Patent Laid-open Publication No. HEI 2-129446. The disclosed automatic transmission achieves 5 speeds (or 6 speeds) with less shift elements, resulting in a compact automatic transmission.
An automatic transmission of the above type usually employs a one-way clutch (xe2x80x9cOWCxe2x80x9d, hereinafter) for permitting rotation only in one direction to facilitate shift control. For example, in the automatic transmission disclosed in Japanese Patent Laid-open Publication No. HEI 2-129446, a set of a coast OWC and a clutch is disposed in parallel with a third speed clutch K4 between a planetary gear unit and a planetary gear set. In Japanese Patent Laid-Open Publication No. HEI 4-29650, a third and fifth speed clutch C3 is likewise provided between a planetary gear unit and a planetary gear set.
Generally, both the transmitting side and the receiving side of a clutch are formed as rotors, and the number of rotor members required in a transmission is an inconvenience owing to large clutch size.
Further, the clutch K4 or C3 disposed between the planetary gear unit and the planetary gear set also serves to prevent relative rotational speed between the shift elements of an upstream planetary gear unit from increasing, due to rotation of the planetary gear set, when the clutch is not participating in a shift operation. That is, rotation of an input shaft is directly input to the planetary gear set without passing through the planetary gear unit. However, in such a structure, when the rotation of the input shaft is input to the planetary gear unit in a shift operation, the clutch K4 or C3 accordingly transmits the rotation that has been reduced in speed, with increased torque, by the planetary gear unit to the planetary gear set. However, the capacity of the clutch K4 or C3 must be increased in accordance with the torque to be transmitted, thus undesirably enlarging the clutch size.
In view of the aforementioned circumstances, it is a first object of the invention to provide an automatic transmission for a vehicle of a type combining a planetary gear unit and a planetary gear set, in which input rotation may be transmitted to the shift elements of the planetary gear set without passing through the planetary gear unit. The automatic transmission according to the invention can be formed into a compact structure by reducing the number of clutches to the extent possible, as well as by reducing the size of the clutch itself.
It is a second object of the invention to provide an automatic transmission for a vehicle of the aforementioned type having a simple and compact structure, which prevents increase in the relative rotational speed between shift elements of a planetary gear unit which is not participating in a shift operation.
According to a first aspect of the invention, there is provided an automatic transmission for a vehicle including an input shaft and an output shaft, a first planetary gear unit, and a planetary gear set having at least four shift elements, in which the planetary gear unit and the planetary gear set cooperate to provide multistage shifting. The first planetary gear unit includes a first shift element to which rotation of the input shaft is input through a first clutch, a second shift element stopped from rotating by a first engagement unit, and a third shift element for outputting, to a fourth shift element of the planetary gear set, rotation that is reduced in speed relative to the rotational speed of the input shaft.
This first aspect of the invention provides a gear train having a first clutch which connects/disconnects the input shaft to/from the first shift element. This gear train has no clutch between shift elements of the planetary gear unit and the planetary gear set. The second shift element is appropriately braked/released by the first engagement unit so as to prevent excessive increase in the relative rotation between shift elements of the first planetary gear unit when not participating in the shift operation as shown in FIG. 13.
Since the first clutch is disposed between the input shaft and the first shift element to which the rotation of the input shaft is transmitted, it is possible to reduce the capacity of the first clutch to be smaller than that of the prior art clutch disposed between the shift elements of the planetary gear unit and the planetary gear set to which reduced rotational speed (thus, greater torque), compared with the rotational speed of the input shaft, is transmitted. Thus, the resultant clutch structure is more compact and the automatic transmission itself can be made more compact.
In a preferred embodiment, the first engagement unit includes a first brake and a first one-way clutch for permitting rotation only in one direction, and the second shift element is connected to the transmission casing and stopped from rotating when the first brake or the first one-way clutch is engaged. The first brake is capable of applying an engine brake (coast) and, therefore, unlike the conventional system, a set of the clutch and the one-way clutch need not be disposed between rotatable elements of the first planetary gear unit and the planetary gear set for coasting, thus simplifying the overall structure and making the automatic transmission more compact.
The first brake is indispensable for preventing the first shift element from rotating at a high speed during idling of the first planetary gear unit. Therefore, the first brake can serve both as the brake that prevents high-speed rotation and as the brake for engine braking. This makes it possible to correspondingly reduce the number of frictional engagement elements. Therefore, the shift control can be easily performed and the automatic transmission can be made into a compact structure.
In another preferred embodiment of the invention, the first shift element is connected to the transmission casing through a second one-way clutch and a second brake, and the second one-way clutch is locked by engaging the second brake so as to stop rotation of a fourth shift element. Thus, in this preferred embodiment of the invention, the fourth shift element is stopped from rotating, with the reaction force borne by the first and second one-way clutches and the second brake (see FIG. 5(b)). Therefore, torque loads on the second one-way clutch and the second brake can be reduced compared with the case where the fourth shift element is directly provided with the second one-way clutch and the second brake (see FIG. 4(b)), thus allowing the automatic transmission to be formed into a more compact structure.
Preferably, the first shift element is connected to the case through a third brake, and the fourth shift element is stopped from rotating by engaging the first brake and the third brake.
In the preferred form of the first aspect of the invention, the fourth shift element is stopped from rotating with the torque borne by the first brake and the third brake. Therefore, the torque load on the third brake can be reduced compared with the case where the fourth shift element is directly provided with the third brake, thus allowing the automatic transmission to be formed into a more compact structure.
According to the preferred embodiment of the first aspect of the invention, the planetary gear set includes four shift elements, the fourth shift element is connected to the third shift element of the first planetary gear unit and can be stopped from rotating by a second engagement unit, a fifth shift element is selectively connected to the input shaft by a second clutch, and can be stopped from rotating by a third engagement unit, a sixth shift element is connected to the output shaft, and a seventh shift element is selectively connected to the input shaft by a third clutch. Thus, the preferred form of the first aspect of the invention provides an automatic transmission with five or six forward speeds in a compact structure, by utilization of the fourth to seventh shift elements.
According to a second aspect of the invention, there is provided an automatic transmission for a vehicle including an input shaft and an output shaft, a first planetary gear unit including one shift element stopped by a first engagement unit, a planetary gear set having at least four shift elements, and a first clutch for transmitting rotation of the input shaft to one of the shift elements of the planetary gear set through the first planetary gear unit, in which one of a plurality of the shift elements is appropriately selected, and rotation of the input shaft is transmitted to the output shaft through the selected shift element so as to achieve a selected speed stage. The first planetary gear unit includes a first shift element to which rotation of the input shaft is input, a second shift element stopped from rotating by the first engagement unit, and a third shift element for outputting rotation at a speed, different from that of the input shaft, to the fourth shift element of the planetary gear set. The first engagement unit includes a first brake and a first one-way clutch for permitting rotation in only one direction. The second shift element is stopped from rotating when the first brake or the first one-way clutch is engaged.
In the second aspect of the invention, the second shift element is stopped from rotating through the first brake and the first one-way clutch disposed between the second shift element and the case. Therefore, the first brake is capable of providing for engine braking. Unlike the conventional system, a set of a clutch and a one-way clutch need not be disposed between the first planetary gear unit and the planetary gear set for coasting. Accordingly, the complicated structure for supporting those mechanisms can be eliminated, thus further contributing to compactness of the automatic transmission.
The first brake is indispensable to prevention of the first shift element from rotating at a high speed during idling of the first planetary gear unit. The first brake can serve both as the brake that prevents high-speed rotation and as the brake for engine braking. This makes it possible to correspondingly reduce the number of frictional engagement elements. Therefore, the shift control can be easily performed and the automatic transmission can be made into a more compact structure.
According to a preferred form of the second aspect of the invention, the first clutch is disposed between the first shift element and the input shaft, the first shift element is connected to the second one-way clutch through a second brake, and the second one-way clutch is locked by engaging the second brake so as to stop the rotation of the fourth shift element.
In the preferred form of the second aspect of the invention, the fourth shift element is stopped from rotating with the reaction force borne by the first and second one-way clutches and the second brake (see FIG. 5(b)). Therefore, the torque load on the second one-way clutch and the second brake can be reduced compared with the case where the fourth shift element is directly connected to the second one-way clutch and the second brake (see FIG. 4(b)), thus allowing the automatic transmission to be formed into a more compact structure.
Also, according to a preferred form of the second aspect of the invention, the first clutch is disposed between the first shift element and the input shaft, the first shift element is connected to a case through a third brake, and the fourth shift element is stopped from rotating by engaging the first brake and the third brake.
Further, in the preferred form of the second aspect of the invention, the fourth shift element is stopped from rotating with the torque borne by the first brake and the third brake. Therefore, the torque load on the third brake can be reduced compared with the case where the fourth shift element is directly connected to the third brake, thus allowing the automatic transmission to be formed into a more compact structure.
In the preferred form of the second aspect of the invention, the first planetary gear unit outputs reduced rotation from the third shift element (R1) when the second shift element (CR1) is stopped from rotating by the first engagement unit.
In the preferred form of the second aspect of the invention, reduced rotation can be input to the planetary gear set in addition to rotation of the dual system input shaft. Therefore, as the number of multi-gear stages increases, it is possible to provide more speed change paths within the planetary gear set as shown in the velocity diagram for the planetary gear set. This gives enhanced flexibility in gear ratios and allows employment of an optimum gear ratio.
In the preferred form of the second aspect of the invention, the planetary gear set includes four shift elements, the fourth shift element is connected to the third shift element of the first planetary gear unit, and the rotation of the fourth shift element at least in one direction, is stopped by the second engagement unit, a fifth shift element is selectively connected to the input shaft by a second clutch, and the rotation of the fifth shift element at least in one direction, is stopped by the third engagement unit, a sixth shift element is connected to the output shaft, and a seventh shift element is selectively connected to the input shaft by a third clutch. Thus, in the preferred form of the second aspect of the invention, an automatic transmission of five or six forward speeds can be formed into a compact structure by utilizing fourth to seventh shift elements.
According to a third aspect of the invention, there is provided an automatic transmission for a vehicle including an input shaft and an output shaft, a first planetary gear unit, and a planetary gear set having at least four shift elements, in which the planetary gear unit and the planetary gear set cooperate to provide multistage shifting. The first planetary gear unit includes a first shift element to which rotation of the input shaft is input through a first clutch, a second shift element stopped from rotating by a first one-way clutch or a first brake, and a third shift element for outputting, to a fourth shift element of the planetary gear set, rotation at a speed that is reduced relative to the rotational speed of the input shaft. The second shift element is stopped from rotating when the first brake or the first one-way clutch is engaged.
In this third aspect of the invention, the first clutch connects/disconnects the input shaft to/from the first shift element without a clutch provided between shift elements of the planetary gear unit and the planetary gear set, and the second shift element is appropriately stopped/released using the first one-way clutch and the first brake so as to prevent excessive increase in the relative rotational speed between shift elements of the first planetary gear unit when not participating in a shift operation, as shown in FIG. 13.
Further, since the first clutch is disposed between the input shaft and the first shift element to which the rotation of the input shaft is transmitted, it is possible to reduce the capacity of the first clutch to be smaller than that of a clutch disposed between the shift elements of the planetary gear unit and the planetary gear set to which rotation at a reduced speed (thus, greater torque), compared with the rotational speed of the input shaft, is transmitted. The resultant clutch structure, thus, can be formed into a more compact structure.
Further, according to a preferred form of the third aspect of the invention, the first clutch is disposed between the first shift element and the input shaft, the first shift element is connected to a case through the second one-way clutch and a second brake, and the second one-way clutch is locked by engaging the second brake so as to stop the rotation of the fourth shift element.
In the preferred form of the third aspect of the invention, the fourth shift element is stopped from rotating with the reaction force borne by the first and the second one-way clutch and the second brake (see FIG. 5(b)). Therefore, the torque load on the second one-way clutch and the second brake can be reduced compared with the case where the fourth shift element is directly connected to the second one-way clutch and the second brake (see FIG. 4(b)), thus allowing the automatic transmission to be formed into a more compact structure.
According to a preferred form of the third aspect of the invention, the first shift element is connected to a case through a third brake, and the fourth shift element is stopped from rotating by engaging the first brake and the third brake.
Also, in the preferred form of the third aspect of the invention, the fourth shift element is stopped from rotating with the torque borne by the first brake and the third brake. Therefore, the torque load on the third brake can be reduced compared with the case where the fourth shift element is directly connected to the third brake, thus allowing the automatic transmission to be formed into a more compact structure.
According to a preferred form of the third aspect of the invention, the first planetary gear unit outputs rotation at a reduced speed from the third shift element when the second shift element is stopped from rotating by the first engagement unit.
In the preferred form of the third aspect of the invention, reduced rotation can be input to the planetary gear set in addition to rotation of the dual system input shaft. Therefore, as the number of multi-gear stages increases, it becomes possible to prove more options in the velocity diagram on the side of the planetary gear set. This, in turn, provides enhanced flexibility of gear ratio and allows employment of a more optimum gear ratio.
According to a preferred form of the third aspect of the invention, the planetary gear set includes four shift elements, the fourth shift element is connected to the third shift element of the first planetary gear unit, and the fourth shift element can be stopped from rotating by a second engagement unit, a fifth shift element is selectively connected to the input shaft by a second clutch, and can be stopped from rotating by a third engagement unit, a sixth shift element is connected to the output shaft, and a seventh shift element is selectively connected to the input shaft by a third clutch. Thus, in the preferred form of the third aspect of the invention, an automatic transmission of five or six forward speeds can be formed into a compact structure by employing the fourth to the seventh shift elements.